On Tue, Sep 20, 2011 at 11:38:15AM +0400, Nikita A Menkovich wrote: I'm guessing this is a problem with the host kernel (Linux?). Specifically you may be saturating SATA, your Southbridge, or PCI if it's using that. Here is the code anyway: http://git.annexia.org/?p=libguestfs.git;a=blob;f=fish/alloc.c;h=7799e4e1... Notice that the 'allocate' command takes the non-sparse code path. What happens next depends on: - does your operating system have the posix_fallocate call? - does your filesystem support efficient fallocate? (ext3: no, ext4: yes, xfs: yes) If the answer to either question is 'no' then you end up in a loop which writes zeroes. For a 20 GB file this is going to be slow and is going to saturate SATA. If the answer to *both* questions is 'yes', then ext4/xfs should be able to allocate the required extents and lazily initialize it. This is typically very fast, almost instantaneous. Probably the best idea is to use ext4, or use 'sparse' instead of 'allocate' (but with 'sparse' you pay the same penalty, just you pay for it later on). Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into Xen guests. http://et.redhat.com/~rjones/virt-p2v